There are no effective treatments for castrate-resistant (CR) prostate cancer (PCa). The work performed during the previous funding cycle provided proof-of-concept for Stat5a/b acting as a critical survival factor for human PCa cells. Inhibition of Stat5a/b leads to extensive death of Stat5-positive human PCa cell lines and blocks prostate xenograft tumor growth in nude mice. We showed that elevated levels of nuclear Stat5a/b in PCa: 1) are associated with high histological grade, 2) predict development of CR recurrent PCa and increased risk of PCa-specific death, 3) are associated with CR PCa and distant metastases and Stat5a/b activation promotes metastatic behavior of PCa cells in vitro and in vivo. Two novel concepts emerged from the work of the previous funding cycle: 1) Synergy between Stat5a/b and androgen receptor (AR)-regulated gene transcription in PCa cells7, 2) Amplification of the Stat5a/b gene locus occurs in up to 30-40% of high grade PCas and 60% of CR PCas. Preliminary progress supports exploration of three independent mechanisms of Stat5-driven growth of CR PCa: i) Stat5a/b up-regulation of full-length AR in PCa cells, ii) Stat5a/b induction of ligand-independent constitutively active AR splice variants, and iii) Stat5a/b stimulation of PCa cell survival independently of AR through Surviving. We will test the central hypothesis: Somatically amplified Stat5a/b promotes CR growth of PCa through AR- dependent and AR-independent mechanisms, and Stat5a/b provides a therapeutic target protein and response-predictive marker for CR PCa. We will pursue three aims: 1) Determine mechanisms of Stat5a/b promotion of castrate-resistant PCa growth. 2) Determine the diagnostic and functional significance of Stat5a/b gene amplification for prostate cancer progression to CR disease and clinical outcome. 3) Determine the efficacy of therapeutic targeting of Stat5a/b pathway in CR prostate cancer. At the completion of this work, we expect to have determined whether Stat5a/b gene amplification, its AR-independent PCa growth promotion and transcriptional co- action with the full length AR and AR splice variants may represent novel mechanisms of CR growth of PCa. This is significant because pharmacological inhibitors of Jak2-Stat5a/b signaling pathway represent rapidly implementable therapeutic strategies for CR PCa. We will determine whether Stat5a/b gene amplification, alone or in combination with AR gene amplification may provide a new predictive marker for identification of patients with PCa likely to progress to CR disease and would benefit from early aggressive intervention. Furthermore, Stat5a/b gene amplification may represent a predictive marker of responsiveness to therapies targeting Jak2-Stat5 signaling pathway. The proposed research may directly impact development of new strategies for improved and individualized therapy for PCa patients.